The present invention relates to charge deflection type ink jet printing apparatus which can accurately adjust the size (height or width) of characters to be printed by controlling the number of so-called "guard drops" during a detection and control of an amount of deflection.
Ink jet printing apparatus are known in the art in which ink is supplied into an ink jet head and subjected to pressure oscillation by a vibrator. This pressure oscillation causes the ink to be ejected from the head and separate into droplets. Where it is desired to print a drop on a sheet of paper, an electrostatic charge is applied to the droplets. The charged droplets are electrostatically deflected by deflection electrodes and hit the paper to print the dot. Where it is not desired to print a dot, the droplets are not charged and are therefore not deflected by the deflection electrodes and hit a gutter instead of the paper.
In many applications, it is desirable to be able to vary the size (height or width) of the characters or other images being printed by the ink jet printing apparatus. This has been done in the prior art by either one of three different methods: moving an ink jet head upwardly or downwardly to vary the direction of ejection of ink droplets, moving a deflection detecting electrode either vertically or longitudinally to vary its position thereof, and moving both the ink jet head and deflection detecting electrode at the same time. All these methods, however, rely on manual operations of an adjusting screw or the like which offers only a limited accuracy to the adjustment.
Another factor on which amounts of deflection of charged ink droplets depends is the number of guard drops, that is, the number of uncharged ink droplets which intervene between adjacent charged ink droplets. The deflection increases as the number of guard drops or spacing between adjacent charged ink droplets is so increased as to substantially eliminate mutual influence between the charged ink droplets. Stated another way, the deflection decreases with a decrease in the number of guard drops. The present invention has been elaborated in view of such a relation between the deflection and the number of guard drops.